Conventional engines include a fuel circuit which conducts fuel with a fuel pump from a fuel tank to the combusters. In general, a separate lubricating oil system is provided to circulate oil between bearings and other moving components and an oil tank through an oil pump, oil filter and optionally through a heat exchanger in a continuous cycle.
To heat the fuel sufficiently to avoid icing of the fuel filter, in gas turbine engines the oil and fuel circuits are both conveyed to a heat exchanger whereby the hot oil withdrawn from the bearings is cooled and cold fuel from the fuel tank is heated simultaneously. Conventionally, the fuel pump and oil pump are mechanical driven by an auxiliary gearbox mechanically connected to a rotating engine shaft. The engine shaft includes a gear which drives a radially extending power take-off shaft to the auxiliary gearbox. The auxiliary gearbox, in addition to driving the oil and fuel pumps, is used to drive a hydraulic pump for the aircraft hydraulic system, connects the starter/generator to the engine shaft and drives the oil/air separator as well as other oil and fuel system components.
The conventional auxiliary gearbox has proven reliable, however, due to the weight of gears and shafts, and the overall mechanical complexity, it represents a significant cost in engine weight and performance as well as increasing the cost of engine assembly and maintenance.
The speed of the auxiliary gearbox is necessarily dependent upon the rotation speed of the engine shaft. There is very little freedom for individual control of driven components and adaptability. If different components driven by the auxiliary gearbox would be optimally driven at different speeds at different times during engine operation, such optimum efficiency is sacrificed in order to keep the auxiliary drive system simple and avoid further weight penalties, or complexity in mechanisms and control systems.
The auxiliary gearbox therefore represents a significant drain on engine performance and it is not necessary the most efficient manner of powering these engine components. The current trend is to eliminate parts or reduce the number of separate parts, improve performance, reduce weight and reduce overall cost of the engine in design, manufacture, assembly and operation.
Regarding conventional oil pumps, the supply of oil is directly dependent on the speed of the engine since the oil pump is driven by the auxiliary gearbox that is driven by the engine shaft. The rotational speed of a conventional oil pump is much lower than the rotational speed at which the engine shaft rotates since oil would cavitate the oil pump otherwise. The stepping down of the rotational speed with mechanical gears involves significant cost in manufacture, maintenance and performance.
Further, there is no direct correlation between the engine rotational speed and the need for oil to cool and lubricate the bearings. During aircraft take-off the engine speed is high, the engine thrust is high. Consequently the load on the bearings and need for oil cooling of the bearings is also high. However, at cruising speed and altitude, the aircraft's airspeed is high and therefore the engine speed is high as well. During cruising, the engine thrust is much lower than at takeoff. Due to the lower thrust and the cooler ambient air temperature at cruising altitude the demand for oil is much lower as well. However, since the conventional oil pump is driven at a rate dependant on the engine speed, unnecessarily large volumes of oil are circulated during cruising. As a result, energy is wasted in pumping the oil, and oil system components are subjected to unnecessary wear. Further, the temperature of oil withdrawn from the bearings is lower, due to shorter residence times within the bearing gallery, and as a result the exchange of thermal energy to the fuel is reduced thereby increasing the risk of ice buildup on the fuel filter.
It is an object of the present invention to eliminate or substantially reduce the dependence on an auxiliary gearbox; thereby improving engine efficiency and reducing weight and cost.
It is a further object of the invention to reconfigure the oil system of a gas turbine engine to combine various components together in a compact easily manufactured unit.
It is a further object of the invention to reduce the amount of energy wasted or unnecessarily consumed in driving the oil and fuel components with an auxiliary gearbox.
Further objects of the invention will be apparent from review of the disclosure and description of the invention below.